Precision pressure manometer

ABSTRACT

A design Bourdon tube manometer, wherein tube deflection is detected by sensing the arc subtended by a light beam reflected from a mirror on the Bourdon tube, has a fixed light source and a null reading reflected beam detector. The Bourdon tube itself is mounted on a mounting member which is selectively movable with respect to a frame member on which is mounted the null reading detector. This permits linearizing the response of the instrument. Initially, the range of the instrument is adjusted by moving the mirror parallel to the source light beam to produce a read-out corresponding to the upper limit of the pressure range desired to be measured. Then the mirror is moved perpendicular to the source light beam to produce a linear response at a point at approximately the mid-point of such range.

United States Patent [191 Fruit [1 3,707,868 [451 Jan.2,1973

[21] Appl.No.: 134,607

[52] U.S.Cl. ..73/4, 73/418 [51] Int. Cl. ..G0ll' 27/00 [58] Field ofSearch ..73/418, 411, 398 R, 4

[56] References Cited UNITED STATES PATENTS 3,067,617 12/1962 Buck..73/41 8 X Primary Examiner-Donald O. Woodiel Attorney-Arnold, White &Durkee, Tom Arnold, Bill Durkee, Jack C. Goldstein, John F. Lynch, Louis'1,

Pirkey, Frank s. Vaden, mind RobiiXTWfiiE [57] ABSTRACT A design Bourdontube manometer, wherein tube deflection is detected by sensing the arcsubtended by a light beam reflected from a mirror on the Bourdon tube,has a fixed light source and a null reading reflected beam detector. TheBourdon tube itself is mounted on a mounting member which is selectivelymovable with respect to a frame member on which is mounted the nullreading detector. This permits linearizing the response of theinstrument. Initially, the range of the instrument is adjusted by movingthe mirto! parallel to the source light beam to produce a read-outcorresponding to the upper limit of the pressure range desired to bemeasured. Then the mirror is moved perpendicular to the source lightbeam to produce a linear response at a point at approximately themid-point of such range.

9 Claims, 12 Drawing Figures SHEET 1 BF 3 PATENTEDJM 1925 FIG. 3

Jerry L. Fruit nvv /v TOR Maw Ma ATTORNEYS =PATENIEOm2 ms' 3.707.868

1 1 i H 59 l 77 w j r a 1" l i 97 5 96 Jerry L. Fruit FIG 4 I INVENTOR111M wide & 7m

- ATTORNEYS PATENTED .1432 I975 sum 3 05 3 u. 3, E Q 5 U READOUT PRESS.

FIG. 9

50 PS/ 3 F1 113 ACTUAL B, E PRESS.

TUAL PRESSURE gig f; 700 R S E d F/G.8B c u 707 v 0 A 100 34 w v PS/\PRESS. SCALE Jerrv I Fruit INVENTOR 700 PSI Ma, mm mm 12 v l G- 3 cATTORNEYS 50Psl PRECISION PRESSURE MANOMETER BACKGROUND OF THE INVENTIONThe instant invention relates to precision pressure measuring devices.More particularly, the instant invention provides a novel Bourdon tubemanometer and further provides a method of adjusting the novel designmanometer to achieve a substantially linear response over a preselectedrange. The method of this invention can be likewise applied to adjustthe linearity of other instruments which have optical readout systemswherein a measurement is effected by measuring the deflection of amirror or the like.

The Bourdon tube has long been known as a pressure measuring device.Basically, a Bourdon tube comprises a length of coiled or twisted tubinghaving a flattened cross section. The tube is formed into the shape of aC- spring, a helix or the like with one end fixed and the other end freeto deflect. A pressure differential introduced across the walls of thetube causes the coiled or twisted tube to change shape as a function ofpressure, and this change of shape can be detected at the free end.

For extreme precision pressure measuring devices, fused quartz Bourdontubes have been found useful. Quartz provides a nearly ideal springmaterial because of its near perfect elasticity and the absence ofhysteresis or creep in quartz springs. Although it will be appreciatedthat the instant invention can be adapted for use with a variety ofBourdon tube pressure gauges, it will also be apparent that a helicalquartz Bourdon tube is preferred in the high precision instruments ofthis invention.

A precision quartz Bourdon tube manometer is disclosed in U.S. Pat. No.3,286,529. In that patent there is described a manometer having anoptical system to detect Bourdon tube deflection whereby a light beam isreflected from amirror attached to the free' end of the Bourdon tube todetermine the amount of deflection of the mirror. This method ofmeasuring tube deflection minimizes frictional and inertial forces.

In the manometer disclosed in the mentioned patent, a source light beamimpinges upon a mirror attached to the free end of the Bourdon tube andis reflected to a light-sensitive detector which gives a null readingwhen the system is in equilibrium. Upon application of pressure acrossthe walls of the Bourdon tube, the tube deflects producing an angularrotation of the mirror, and a suitable mechanism drives thelight-sensitive detector to once again intercept the reflected beam atthe tive to the photocells intercepting the reflected beams Hence, whenthe instrument is reading null, the light beam is always perpendicularto the mirror.

Despite theadvantages of quartz as a spring material for precisionmanometers, the response of a quartz helix Bourdon tube over typicalranges desired to be measured, e.g., 0-100 psi, is slightly nonlinear.Other materials also give non-linear responses. Nonlinearity of springmaterials results from the fact that as various spring materials aredeformed by pressure, they may become more resistant or less resistantto additional deformation by equal additional increments of pressure.

Consequently, it has heretofore been necessary in the art to calibrateeach pressure instrument individually against highly accurate referencepressures and to provide a correction chart with each instrument so thatthe operator could apply correction factors necessary to translateinstrument response to true pressure at each readout pressure in therange to be measured.

It would accordingly be highly desirable if there could be provided ahighly accurate Bourdon tube manometer which would also demonstrate asubstantially linear response within acceptable limits of error.

SUMMARY OF THE INVENTION There is accordingly provided by this inventiona novel Bourdon tube pressure manometer which utilizes an opticaldetection system and a fixed light source, which manometer maybeadjusted to provide a substantially linear readout over a preselectedrange.

There is further provided in accordance with this invention a novelmethod of adjusting such a manometer or a like operating instrument toproduce a substantially linear response over such a preselected rangewithin acceptable'limits of error.

The novel manometer of this invention utilizes a helical Bourdon tubepressure gauge having a mirror mounted thereon adapted to deflect uponapplication of a pressure differential across the walls of the Bourdontube, a light sensing system for measuring the angular deflection of themirror by detecting the arc subtended by a light beam reflected from themirror upon deflection. The light sensing system includes a light sourcefixed with respect to the deflecting mirror on the bourdon tube whichlight source directs a light beam at the mirror, and a photosensitivelight detector which is adapted to rotate to intercept the reflectedlight beam at null reading position.

In a further important and novel structural feature, the assembly of theinstrument is such that the position of the mirror with respect to thephotosensitive means can be adjusted. The adjustable, feature permitschanging of the distance between the mirror and the light sensing systemin directions parallel to and perpendicular to the source of light beam.By changing this distance the size of the arc subtended by the beamreflected from the mirror can be changed while the source beam remainsstationary, and the instrument can be adjusted as provided below to givea substantially linear response.

Structurally, the novel manometer of this invention comprises a frame, ahelical pressure gauge having a fixed endand a free end with a mirrormounted on the free end, a first means mounting the helical pressuregauge with respect to the frame, a light source fixedly mounted on saidfirst means for impinging a source light beam on the mirror, at lightsensing system, rotatably mounted with respect to the frame fordetecting the length of the circular arc subtended by a reflected beamfrom the mirror upon application of pressure across the pressure gauge,and means mounting said first means with respect to said frame to permitselective movement of the pressure gauge and mirror, and the lightsource relative to the sensing system, to adjust the size of the arcsubtended by the reflected beam from the mirror, such movement being indirections substantially parallel to the source light beam andperpendicular to the source light beam.

The novel method of this invention involves calibrating such a fixedlight source Bourdon tube manometer for a substantially linear responseover a preselected range, which method involves applying to the Bourdontube pressure gauge a first reference pressure corresponding to theupper pressure of such preselected range; detecting the arc subtended bythe reflected beam from the reflector means mounted on the Bourdon tubepressure gauge as a result of such first reference pressure, moving themirror or other reflector means on the Bourdon tube parallel to thesource beam to change the size of the arc subtended by the reflectedbeam to produce a readout corresponding to the first referencepressures, applying to the Bourdon tube a second reference pressure atwhich the readout differs substantially from the second referencepressure and moving the mirror transversely to the source beam toproduce a readout corresponding to the second reference pressure, suchmovement being effected as to not change the size of the arc subtendedby the Bourdon tube pressure gauge while under such first referencepressure. If the instrument is to be as an absolute pressure measuringdevice, then the first pressure applied will be the lowest reducedpressure desired to be measured by the device. The object is to applyinitially a differential pressure across the Bourdon tube whichcorresponds to the pressure differential which it is desired willproduce a full scale response.

It may be stated that this method of calibrating a nonlinear respondingpressure gauge is accomplished essentially by introducing an additionalnonlinear factor into the system by movement of the mirror. This is notto suggest that the mirror adjustment can be made to compensatecompletely for the nonlinearity of the Bourdon tube pressure gauge atall pressures, but as will become apparent from the followingdisclosures, the adjustments made in accordance with the novel method ofthis invention will enable one to reduce the nonlinearity of theresponse of an instrument such as described herein to within acceptablelimits.

The adjustments provided by the novel method of this invention cannot bemade utilizing the prior art device of the patent mentioned above. Inthat apparatus, the light source moves with the light sensitive detectorso that at the null reading position, the reflected beam is essentiallya direct reflection from a mirror perpendicular to the source beam. Withsuch a device, mirror adjustment as provided herein does not enablelinearization of the instrument.

BRIEF DESCRIPTION OF THE DRAWINGS The instant invention will be moreparticularly understood by reference to the accompanying drawings whichillustrate specific embodiments of the invention.

FIG. 1 is a perspective schematic view showing the essential operatingcomponents of a Bourdon tube manometer in accordance with a specificembodiment of this invention.

FIG. 2 is a sectional view of the helical pressure gauge along line 2-2of FIG. 1 to show the disposition of the mirror with respect to thepressure gauge. FIG. 3 is a transverse cross-sectional view of thepressure gauge showing its flattened cross section.

FIG. 4 is an elevational view of a portion of a manometer apparatus inaccordance with a specific embodiment of this invention.

FIG. 5 is a top view of the portion of the manometer apparatus shown inFIG. 4.

FIG. 5A is a front view of the apparatus shown in FIG. 5 depicting thedetail of the gear arrangement.

FIG. 6 is a top view of the light sensitive detector taken along line 66of FIG. 4.

FIG. 7 is a sectional view of the light sensitive detector along line7-7 showing the path of the source beam and reflected beam in typicaloperation of an embodiment of this invention.

FIGS. 8A, 8B, and 8C are schematic drawings illustrating how substantiallinearization of a novel manometer in accordance with this invention maybe accomplished in accordance with the novel method of the invention.

FIG. 9 is an illustration of a graph showing the nonlinearity ofresponse of a manometer device after range adjustment in accordance withthis invention has been made.

DESCRIPTION OF THE SPECIFIC AND PREFERRED EMBODIMENTS Referringgenerally to FIG. I, there is illustrated schematically a Bourdon tubemanometer in accordance with the preferred embodiment of this invention.Generally illustrated in FIG. 1 is a sealed Bourdon tube at 12 which isprovided with a mirror 27 at the lower free end thereof. A light beam Adirected at the mirror from a fixed light source 33 is reflected fromthe mirror, and the reflected beam B is intercepted by a photosensitivedetector 30 on a rotatable table 37. Table 37 is driven by a suitableservo mechanism to both intercept the reflected beam and to produce areadout on digital counter 67 corresponding to the arc subtended by thereflected beam.

More specifically, with reference to FIG. 1, Bourdon tube 12 is ahelical spring member, preferably quartz, having a flattened crosssection as illustrated at FIG. 3. Bourdon tube 12 is mounted withinsealed chamber 13 which is composed of end pieces such as 15 (the lowerend piece is not shown) in a pressure-sealing relationship withtransparent cylinder wall 17 which is typically made of glass. Asuitable pressuretight seal may be effected using O-rings or the like ina manner that will be appreciated by those skilled in the art. 7

Within the sealed chamber 13, the upper end of Bourdon tube 12 is heldin fixed position by and is flowconnected with Bourdon inlet tube 21which extends through the wall of end piece 15 and is desirably providedwith a suitable pressure fitting (not illustrated) to maintain thepressure-tight integrity of cyliner 13. Similarly, tube 23 ispressure-fit through end piece 15 and communicates with the volumeoutside the Hourdon tube and within cylinder 13.

Thus, the pressure to be measured may be applied to the interior ofBourdon tube 12 while the exterior of the tube is subjected to areference pressure (which may be a vacuum or a pressure up to 150 psi ormore) or to ambient atmospheric pressure within the sealed chamber 13through tube 23. Since the Bourdon tube will respond by rotating itsfree end to a pressure differential across its walls, the Bourdon tubemay also be used as an absolute pressure measuring device, preferably bypermanently evacuating the Bourdon tube through tube 21 and applying thepressure to be measured to the exterior of the Bourdon tube withinchamber 13 through tube 23. Typically, an oppositely wound Bourdon tubeis used in absolute pressure devices to accommodate the readoutmechanism of the instrument and facilitate interchangeability of Bourdontubes.

As may be seen in FIG. 2, upon application of a pressure differentialacross the walls of the Bourdon tube, the lower end of the Bourdon tubemoves in an arcuate path through an angle 0 which is functionallyrelated to the pressure differential. The rotation of the end of theBourdon tube through angle 0 is a factor which is measured by themanometer apparatus. Accordingly, mirror 27 is mounted on a suitablestabilizing rod 29, advantageously also composed of fused quartz, whichrod is freely rotatable on quartz filament hinges 35 and 36. Thesehinges may be formed by application of localized heat to the rod to drawdown these portions to a filament of reduced diameter to permitvirtually frictionless rotation. A support rod 39 is affixed at itsupper end to Bourdon inlet tube 21 and suspends stabilizing rod 29. Thelower end of the Bourdon tube is coupled to the rod 29 by means ofradial member 25, also composed of quartz so that the movement of thelower end of the Bourdon tube causes the rod to rotate on its axis andproduce a deflection of the mirror.

As a result of the mounting shown, the entire Bourdon tube apparatus ismounted from end plate 15, thus facilitating the insertion of the entireBourdon tube apparatus within sealed cylinder 13. Removable fittings, ofcourse, would be used 'to seal the Bourdon tube within the cylinder.Although these are not illustrated, they can be readily fashioned bythose skilled in the art.

It will be noted from FIG. 2 that an angulardisplacement of the free endof the Bourdon tube by an angle 0 will produce a corresponding angularrotation of mirror 27. But as will be apparent hereinbelow, inasmuch asthe angular rotation of mirror 27 is measured by the reflection of alight beam impinged upon mirror 27, a rotation of the mirror by anamount corresponding to angle 0 will result in reflected beam B beingreflected at an angle from the source light beam Av Thus, the "output inthe form of mirror deflection is multiplied thus increasing thesensitivity of the device.

The angular movement of mirror 27 is detected and measured by amechanism including light source 33 and a series of photocells 30. Thelight source 33 is fixedly mounted with respect to the frame of the unitso that it does not rotate with the mirror or the photocells. Themounting details for light source 33 will be further set forth below.

Advantageously, light source 33 suitably includes a combination ofcondensing lenses to collimate the source light beam to produce awell-defined spot or bar a circle extending in a curved fashion onrotatable table 37. The lightsensitive cells are preferably photovoltaicsilicon cells which are matched for sensitivity and temperaturecharacteristics. The cells at one end of the bank of photosensitivecells, such as at 32, produce a positive signal when impinged by a lightbeam, while the cells at the opposite end 31 produce a negative signal.

The cells, as indicated above, are secured to rotary table 37 which ismounted to the frame of the unit by a bearing arrangement so that itwill rotate about axis which is directly below rod 29 and the center ofrotation of mirror 27 and corresponds to the axis of the Bourdon tube.The table is a pie-shaped segment supporting a bank of photocellsdefining an angle of about 1 10 since it is desirable that mirror 27 notrotate more than about 50 or so for full scale pressure measurement,though this angle of deflection is not at all critical to the operationof the device. Theouter edge of rotatable table 37 provided with gearteeth 41, which gear teeth engage a worm 43. The worm is mounted onshaft 45 which passes through bearings 47 which are suitably mounted tothe frame of the apparatus. The upper end of the shaft as illustrated isconnected through a reduction gear arrangement 49 to servo motor 51.Servo motor 51 is in turn controlled by servo amplifier 53 which has asits input the differential output of the photocells in bank 30.Accordingly, when a pressure is applied to Bourdon tube 12, thedeflection of the mirror will produce a differential output from thephotocells which will cause servo motor 51 to drive rotatable table 37until the impinging light beam again produces a null reading, i.e., adifferential output of zero from the photocells.

In the condition of zero pressure differential across the walls of theBourdon tube, the mirror may be perpendicular to impinging light beam A.Alternatively, the rest position of the mirror may be at an angle to theimpinging light beam such that the perpendicular condition of the mirroroccurs at about mid-range of the instrument. As the pressuredifferential is applied across the Bourdon tube, mirror 27 rotates to anangle 0 from its original position and, while the impinging source lightbeam A remains fixed, the reflected beam B rotates to an angle 20 fromthe original reflected beam. The reflected beam subtends an are which isequal to 20 radians. The reflected beam is intercepted by photocells 30at a point displaced from the null reading position causing adifferential output to be transmitted through servo amplifier 53 andfurther causing servo motor 51 to drive table 37 to once again drive thephotocells to produce a null reading. The lower end of shaft 45 asillustrated in FIG. 1 is accordingly linked to a digital output scale 67which produces a digital readout corresponding to the angle 20 subtendedby the reflected light beam. A suitable transformer or solid statecircuitry is used to create a constant voltage input to the lamp toregulate the intensity of the beam from lamp 33 to keep the lightintensity therefrom substantially constant. Light intensity variationsresulting from voltage fluctuations can produce changes in the nullreading position and introduce error into measurement. By controllinglight intensity as well as utilizing matched photocells, the possibilityof error in this regard is substantially reduced.

With reference now to FIGS. 4 and 5, the structure of the precisionmanometer apparatus in accordance with this invention is shown ingreater detail. FIG. 4 is a side view of a portion of the apparatus ofthis invention illustrating the means for positioning the Bourdontubecontaining cylinder 13 within the entire apparatus. Cylinder mount55 is fixedly mounted on upper support plate 57. In turn, support plate57 is anchored with screws 59 to the upwardly extending support arms 61of frame 60. Suitable lock washers or the like (not shown) are desirablyprovided on screws 59. The holes through upper support plate 57 forscrews 59 permit limited movement of the support plate with respect tothe frame 60. A total movement of about twenty to fifty-thousandths ofan inch in each direction is generally sufficient. Captive screws 63extending upwardly through end plate 15 of the cylinder assembly securethe Bourdon tube cylinder 13 in cylinder mount 55 permitting thesubstitution of one cylinder for another. Varying Bourdon tube springsare available to give pressure responses within different ranges, i.e.,0l00 psi, 0l,000 psi, etc. However, it will be understood that followingcalibration of the instrument as described hereinbelow, cylinders may besubstituted but the replacement cylinders will not produce directreadings.

Light 33 is mounted in fixed position on the underside of support plate57 so that the limited movements of the support plate, as will beexplained below, result in a similar movement of the light source, thuspreserving the position of the light source with respect to the mirror27 at all times. In the method of this invention, it will be understoodthat movement of the light source around a fixed mirror would effect aresult similar to movement of the mirror relative to a fixed lightsource. In the manometer, however, such a construction is not preferred.

Light 33 is held in a suitable positioning means 67 which directs thelight beam at an angle downwardly toward mirror 27. Collimating lenses71 are positioned in the forward part of means 67 to focus the beam oflight at the distance ofthe photocells.

Housing 73 is mounted atop upper support plate 57 and may contain avoltage source for lamp 33 and a servo motor 51. The servo motor drivespulley 75 and contains chain 77 which in turn drives shaft 45. Asdescribed above, shaft 45 carries worm 43 which drives gear plate 37 tobring the photocells to a null position upon rotation of the mirrorfollowing an applied pressure.

Heater 51 surrounds cylinder 55 for purposes of controlling the heat ofthe Bourdon tube pressure gauge within cylinder 13. Any suitable heatersuch as a wire resistance heater around the exterior of cylinder 13 orthe like may be used. While the temperature effect on fused quartz issmall and less than a similar effect on other materials which might beused to measure pressure (e.g., the effect on quartz is less than theeffect on the density of mercury in a mercury manometer), the errorintroduced by temperature effect cannot be considered negligible becauseof the extreme precision of the instrument. Accordingly, it is desirableto control the temperature of the Bourdon tube manometer by means ofasuitable closed loop temperature controlling circuit which uses acontrolling thermistor or the like. Heater circuit 70 is mounted onsupport arms 61.

The temperature of the quartz Bourdon tube may be measured with amercury-in-glass thermometer 81, a thermistor probe or the like. Thethermometer illustrated is maintained in a heat conductive envelope 83extending from end plate 15. The bulb of the thermometer is preferablyplaced proximate the exterior of the Bourdon tube. When measurements aretaken, the temperature of the quartz Bourdon tube is accordingly easilymeasured and for greatest accuracy, pressure measurements should betaken at a temperature corresponding to the calibration temperature ofthe instrument.

Gear plate 37 is provided to suitably rotate with respect to frame 60 bymeans of pin 93 which extends through plate 37 and is fastened to theframe 60 by bolt 95 which is bearing mounted at 96. Table 97 fixedlymounted to and disposed immediately beneath gear plate 37 is providedwith bearings 98 about its periphery to permit smooth rotation of gearplate 37.

When chain 77 from the servo motor drives shaft 101, as seen in FIG. 5A,gear 99 provides a reduced rotation to shaft 45 through gear teeth 103.Shaft 101 is provided with bearings 48 as it is mounted through frame 60similarly to the bearings 47 provided with respect to shaft 45. Bevelledgear 81 at the end of shaft 101 engages bevelled gear 83 which in turndrives counter 67. Spring 105 is maintained on shaft 101 behind stop106, accordingly enabling the operator to disengage gear 81 from gear 83by pushing inwardly on shaft 101. This permits setting of counter 90 tozero for any given position of plate 37.

Since counter 90 responds only to the number of revolutions of gear 83,it will be understood that the full scale count of the instrument can bevaried by simply varying the gear ratios between gears 81 and 83. Forexample, an instrument having a range of 0 to 100 psi or of 0 to 10 psiwould be set up to produce 100,000 counts on counter 90 at full scale.On the other hand, an instrument having a range of 0 to 200 psi wouldmore desirably produce 200,000 counts on the counter at full scale. Thenumber of counts on the counter produced by a given rotation of gear 81can be altered by altering the gear ratio between gear 81 and gear 83.

With reference now to FIG. 6, the photocell assembly 30 is shown. Thephotocells such as 31 and 32 are mounted on stand 111 which has chordalsurfaces facing inwardly toward mirror 27. Conductive band 109 is aprinted circuit strip which is fastened to photocell stand 111 by meansof screws 110 and is electrically connected to the photocells. Suitablebrackets 113 are provided on the interior of stand 111 to enablefastening of the stand and the photocell assembly to plate 37.

With reference now to FIG. 7, the photocell assembly is shown in sectionindicating the relative position of the mirror to the photocells. Itwill be noticed that mirror 27 at the end of the Bourdon tube 12 ispositioned slightly above the plane of the photocells so that thedownward inclined beam from light source 33 will strike the photocellssquarely. The angle between the incident beam from light source 33 andthe reflected beam in the vertical plane has no significance in themeasurements of this invention or in the calibration procedure explainedbelow. Accordingly, although it will be understood that the reflectedbeam defines an angle with respect to the incident beam from lightsource 33 in a vertical plane, this angle in the vertical plane has noeffect on the measurement taken with the novel manometer of thisinvention.

Normally, in the construction of quartz manometer Bourdon tubes, it hasbeen the practice to align the center of rotation of mirror 27 with theaxis of the Bourdon tube 12 and to dispose the center of the mirror andthe axis of the Bourdon tube directly above the center of rotation ofplate 37 indicated in FIG. I at 100. It has further been the practice todispose the photocells so that they substantially define the arc of acircle, such as circle 115 shown in FIG. 6 about the point 100. By thisarrangement, the arc subtended by the reflected beam from the incidentbeam striking the mirror will have a length equal to 26 radians andaccordingly that are subtended will be a direct measurement of the angleof deflection of the mirror upon the application of a pressuredifferential across the Bourdon tube. But, as discussed above, theresponse of the Bourdon tube is not linear and consequently correctionfactors must be applied in order to make the pressure manometers directreading instruments.

The novel manometer of this invention is so constructed that theresponse of the instrument can be calibrated to be substantially linearover its entire range within acceptable limits of error. This isaccomplished by moving the mirror away from the center of the are aroundwhich the photocells rotate.

The novel manometer of this invention is constructed so that sealedcylinder 13 containing Bourdon tube 12 is mounted solely on supportplate 57 whereas the photocell assembly and plate 37 are fixedly mountedwith respect to frame 60. Accordingly, by movement of support plate 57with respect to frame 60, the Bourdon tube mirror 7 may be moved in anydirection thereby displacing the axis of the Bourdon tube and the centerof the mirror from center of rotation 100 of the support plate 37. Sincelight source 33 is also mounted with respect to support plate 57, thisadjustment does not in any way effect the relationship between thesource of the incoming light beam andthe mirror.

With reference now to the FIGS. 8A, 8B, and 8C, the method by which thenovel manometers of this invention may be calibrated to givesubstantially linear response will be illustrated. With particularreference to FIGS. 8A through 8C, there is illustrated schematically theresponse which would be generated by one type of manometer in accordancewith the instant invention. Manometers in accordance with the instantinvention may be absolute (in which case the Bourdon tube is evacuated)or bi-directional in which case their deflection would indicate adeparture of a pressure to be measured above or below a referencepressure. In the manometers schematically illustrated in FIGS. 8Athrough 8C, a light source 33 is disposed behind a bank of photoelectriccells illustrated schematically at 120. Since the position at which thereflected beam from the mirror intercepts the photocells is directlyrelated to the readout on the counter 90, in these figures, thearrangement of photocells has been represented as a readout scale.indicates the geometric center around which the photoelectric cellsrotate.

In FIG. 8A, when impinging light beam A strikes the mirror disposeddirectly over point 100 at zero pressure, reflected beam B is reflectedfrom the mirror and would be intercepted by the photocells at point r.Thus, point r would correspond to zero actual differential pressure andby virtue of the zero adjust features of the manometer, could beadjusted to reflect a reading of zero pressure. However, assuming a0-100 psi range for the Bourdon tube illustrated in FIGS. 8A through 8C,at 100 pounds pressure differential across the Bourdon tube, the mirroris deflected in a counterclockwise direction and produces reflected beamB which is intercepted by the photocells at point t. However, becausethe range has not been adjusted, the arc subtended between point r andpoint it produces a reading which does not correspond to I00 psi, butrather corresponds to a reading possibly somewhat greater than I00 psi,say l00.50 psi. In such an instance, it can be appreciated that in orderto produce a reading of I00 at an actual pressure of 100 psi, thereflected beam at I00 psi would in fact have to subtend a lesser arcthan the arc rt, and be intercepted by the photocells at point u. Toproduce a deflection necessary to subtend the arc ru, it is necessary toreduce the pressure to less than 100 psi. A similar nonlinearity wouldexist at 50 psi where, for example, reflected beam C would subtend arcrv and give a reading somewhat greater than 50 psi. This is thephenomenon observed in virtually any other type of Bourdon tubemanometer.

Initially, in calibrating the instruments in this invention, it isdesired to adjust the range of the instrument, i.e., provide that theinstrument reads properly at the limits of the range to be measured, sothat at zero pressure a reading of zero pressure will be obtained fromthe instrument and that at I00 psi, if such is the range, a reading ofI00 psi will be obtained from the instrument.

The instrument will produce a reading of 100 psi if the reflected beamintercepts the photocells to subtend an are equal to arc ruQSince arc ruis smaller than the are actually subtended by the reflected beam when100 psi is across the instrument, this adjustment of range can beaccomplished by moving the mirror along the light beam toward thephotocells which intercept the light beam. As the mirror is movedforwardly, the length of the arc subtended by the reflected beams B andB is shortened while the angle of deflection produced by these variouspressures remains the same. Thus, for example, in FIG. 8B, the center ofBourdon tube is moved toward the photocells by a distance d to point 101by an amount sufficient so that the arc subtended by the reflected lightbeams from the mirror is equal to the arc ru. As the center of themirror is moved forward by a distance d such that the deflectionproduced by 100 psi will also produce a total deflection subtending anare equal to ru, the instrument will be linear at 0 psi and at 100 psidespite the fact that the mirror is no longer over the center ofrotation of the photocells. In FIG. 8B the same impinging light beam Aat 0 psi yields reflected beam B which is intercepted at point w by thephotocells. The zero adjust mechanism on the manometer permits theoperator to adjust the readout to be zero at point w as easily as atpoint r and consequently the instrument gives a proper readout at thispoint. At 100 psi, reflected beam B would be intercepted by thephotocell at point x such that the entire arc wx subtended between psiand 100 psi is equal to the arc ru. Since the magnitude of theinstrument readout is dependent upon the size of the arc subtended bythe photocells to intercept the reflected beams, the adjustment of themirror by distance d will produce a reading of 100 psi when reflectedbeam 8,, is intercepted at point x. The instrument is therefore adjustedfor linear response at the extremes of the range, i.e., at zero pressureand at 100 psi the instrument reads correctly.

To increase the range, the adjustment is made by rearward movement ofthe mirror. Thus, if originally a 100 psi reference pressure gives areading less than 100 psi, a rearward adjustment would be indicated.

However, inasmuch as this adjustment takes place by merely moving themirror parallel to incoming beam A, the movement has very little effecton the nonlinearity of the instrument in the mid-range. In FlG. 8B, theapplication of 50 psi still gives a reading greater than 50 psi as shownby reflected beam B although 100 psi actual pressure produces a 100 psireading. In fact, if a plot is made of actual pressure againstinstrument response such as shown in FIG. 9, it will be seen that theinstrument reads correctly at 0 psi and at 100 psi, but that theinstrument is substantially nonlinear in the mid-range. Most commonly,the greatest nonlinearity, i.e., the greatest difference between actualpressure and readout pressure exists at about the middle of thepreselected range, between about 40 and 60 percent of the full scalereading. Graphically, in FlG. 9, dotted line OPL represents asubstantially linear response whereas the curve OQL represents actualresponse. It can be seen that the maximum differential m exists in theneighborhood of the mid-range, at about 50 psi. This nonlinearity ofresponse might exist in other areas and be biased toward one end of therange.

After the instrument is adjusted for a range by moving the mirrorforwardly or rearwardly and parallel to the light beam, the instrumentis further adjusted for accurate readout in the mid-range by moving themirror transversely to the incoming light beam as shown in FIG. 8C.Thus, for example, by further adjusting the mirror slightly to the leftto point 102 as shown in FIG. 8C, the instrument can be substantiallylinearized in the mid-range. The transverse movement of the mirror isundertaken and accomplished while simultaneously preserving the lengthof the arc subtended at full scale. Accordingly, in FIG. 8C, it can beseen that at 0 psi impinging light beam A reflects beam 8,, which givesa readout ofO psi, at 50 psi reflects beam 8, which gives a readout of50 psi, and at 100 psi reflects beam B which gives a readout of 100 psi.

It will also be apparent to those skilled in the art that as the mirroris moved transversely in order to obtain a linear readout in themid-range, it may be necessary to further move the mirror slightly in adirection towards the light sensitive detector while moving ittransversely, in order to preserve the size of the arc subtended at theupper limit of the range. Inasmuch as transverse movement displaces thecenter of rotation of the mirror off of a radius around which table 37rotates, it will be understood that any transverse movement generallymight slightly change the size of the arc which would be subtended atthe upper limit of the range. The mirror is therefore ideally movedtransversely along an arcuate path 104. However, with the amount oftransverse movement here referred to, the amount of change is minimal.Upon acquisition of experience in linearizing instruments in accordancewith this invention, both movements may be effected simultaneously andany additional movement parallel to the light beam which might desirablybe accomplished together with the transverse movement of the mirror isaccomplished in the same operation.

In actual practice, linearization of the instrument in accordance withthe method of this invention is accomplished by measuring the actualrange of the instrument and adjusting the size of the arc subtended toproduce a response on the instrument equal to the desired range.Movement of the mirror in this adjustment is away from the light sourceto increase the range and toward the light source to decrease the rangeof the instrument. Generally, an adjustment of 0.020 inch is sufficientto change the range by an amount corresponding to 0.15 percent of thefull scale reading (0.15 psi in the case ofa psi range).

Following the adjustment of the range, the linearity of the instrumentis measured, typically by preparing a plot such as illustrated in FIG.9. The mirror is adjusted in a direction transverse to the originallight beam in order to obtain a linear response in the most nonlinearregion as may be determined from the plot. Generally, a movement ofabout 0.030 inch perpendicular to the light beam A will result in achange in reading of about 0.4 percent of full scale reading in themidrange of the instruments of the type described. The mirror adjustmentis effected by adjusting plate 57 which moves the mirror and lamp.

The adjustment is accomplished by loosening screws 59 which hold supportplate 57 to frame 60. Preferably, some type of adjustment mechanism isclamped to the leg 61 of frame 60 and the movement of the support platemay be accomplished by insertion of a series of measured pieces, in thenature of feeler gauges to effect the desired movement. The devices ofthis invention are nonlinear by an amount corresponding to about 0.5percent of full scale. The range adjustment with a quartz helix varieswith each spring member and consequently it may be necessary to increaseor decrease the range by moving the mirror away from or closer to thephotocells. However, the deflection of quartz helix at about 50 percentof full scale is almost usually too much, and accordingly compensationto linearize this response is almost invariably negative (i.e., movementof the mirror toward the zero side of the scale). But if the response ofthe Bourdon tube at 50 psi is low, an adjustment to the higher side ofthe scale would be indicated.

It is pointed out that the linear response referred to herein is alinear response between the reading of the instrument and the actualcondition being measured. In all cases it would be desirable to achievesuch a readout so that an instrument could be made direct reading. Itwill therefore be recognized that the method of this invention may beemployed to generate a direct response of an instrument to a nonlinearparameter to be measured.

Once support plate 57 has been positioned bymeans of feeler gauges orthe like to give substantially linear response, screws 59 are tightenedwith suitable loc'k washers in place in order to lock the support'platewith respect to frame 60; If desired, however, lockable vernier screwscould be provided to mount the support plate 57 on frame 60 and therebypermit the establish ment of various positions for linear response ofvarious Bourdon tubes thereby'permitting the operator to interchangeBourdon tubes in the manometer.

The method of this invention may be practiced in other ways as will beobvious to those skilled in the art. Upon initial calibration of themanometer in accordance with this invention, an operator, cognizant ofthe novel method of this invention, can begin adjustment of the Bourdontubeby moving it simultaneously forwardly or rearwardly of the center ofrotation of the photocells and simultaneously adjust the Bourdon tubetransversely with respect to that center of rotation. Adjustments mustbe made on a trial and error basis, and perhaps several adjustments bothparallel and transverse to the light beam might be required. Onceexperience is obtained by adjusting a Bourdon tube manometer inaccordance with the method of this invention, it will become possiblefor an operator to estimate fairly closely the direction and extent ofmovement which will be necessary to achieve substantial linearity in theinstrument.

With respect to the novel manometer construction of this invention, itwill be apparent to those skilled in the art that a number of variousservo mechanisms, drive systems, pressure fittings and the like may beadapted to manometers in accordance with this invention, and that thedetails of construction of the embodiments shown are not critical.Indeed it is only necessary in accordance with this invention to providethe Bourdon tube mount with a means whereby it is adjustable withrespect to the center of rotation of the photocells. It is alsodesirable in accordance with this invention to similarly mount the lampdirecting the source beam at the Bourdon tube mirror with the Bourdontube so that this relationship is preserved.

It is further pointed'out that generally once the range of theinstrument has been established, it is typically necessary only tolinearize the instrument at one additional point in order to obtain anoptimum direct reading response over the entire preselected range.Further attempts to linearize the response of the instrument atadditional points while trying to maintain the linear response of theinstrument at the zero point, the upper limit ofthe range, and at themid-range is extremely difficult and generally should not be attempted.

Although the method of this invention has been specifically discussedwith respect to a Bourdon tube pressure measuring device, it will beapparent that a number of instruments having optical readout systems canbe calibrated for linear response in a similar fashion.. Thus, forexample, temperature measuring devices can utilize bimetallictemperature-sensitive spirals. An optical readout whereby the arcsubtended by a light beam reflected from a mirror suspended from Hence,this invention comprehends adjusting the range and calibrating any suchdigitally reading instrument for a linear response over a preselectedrange. It is necessary only that the instrument have a transducer, i.e.,a member which provides a physical response functionally related inmagnitude to the condition to be measured (be in temperature, pressure,electromag netic field, etc.) to produce a deflection of a reflectormeans, and includes a lightsource for directing a beam of light at thereflector, a light sensor means for detecting the arc subtended by thereflected beam. The readout of such a measuring instrument can becalibrated for substantially linear response over a preselected range ofvalues measuring the condition. The method is accomplished by initiallyapplying to the transducer a first reference condition of predeterminedmagnitude corresponding to the upper limit of said range, detecting the.arc subtended by the reflected beam from the reflector means as a resultof application of said first reference condition, moving said mirrorparallel to the source light beam to change the size of the arcsubtended by the reflected beam intercepted on the light sensor toproduce an accurate readout corresponding to said first referencecondition, applying a second reference condition of predetermined magnitude within said preselected range at which the readout differssubstantially from the magnitude of the second reference condition, andmoving said mirror transversely to the direction of the source beamwhile preserving the size of the arc subtended by the first referencecondition to produce an accurate readout at said second referencecondition. The application of this method to various instruments havingreflected-beam optical response systems including those wherein, mirrordeflection is produced by a deflecting spiral, by a twisting filament orthe like will be understood by those skilled in the art. The method ofthis invention is useful in calibrating those instruments havingtransducers which produce nonlinear response to the condition to bemeasured and from which a substantially linear response is desired. If acompletely linear-responding transducer can be utilized, of course, thedevice can be made direct reading by conventional methods. On the otherhand, if an instruments response is not substantially linear, e.g., ifthe transducer has an exponential response, linearization over asignificant, range cannot be achieved.

What is claimed is:

1. In a Bourdon tube manometer having a reflector means mounted on aBourdon tube pressure gauge, a mirror deflection sensing means includinga light source for directing a source beam of light spaced from saidreflector means at said reflector means and a light sensor for detectingthe arc subtended by the reflected beam from such reflecting means, anda readout means for producinga digital readout of pressure related to 6the bimetallic element would operate similarly to the manometerdescribed and could be similarly'adjusted for linearity.

the arc subtended between said source beam and said reflected beam, themethod of calibrating such a manometer for a substantially linearresponse over a preselected range which comprises:

applying to such Bourdon tube pressure gauge a first reference pressurecorresponding to the upper limit of such preselected range; detectingthe-arc subtended by the reflected beam from such reflector means as aresult of application of such first reference pressure;

moving said reflector means parallel to said source beam to change thesize of the arc subtended by said reflected beam on said light sensor toproduce an accurate readout of said first reference pressure;

applying to said Bourdon tube pressure gauge a second reference pressurewithin said range wherein said readout differs substantially from saidsecond reference pressure; and

moving said reflector means transversely to the direction of said sourcebeam while preserving the size of the arc subtended at said firstreadout pressure to produce an accurate readout corresponding to saidsecond reference pressure.

2. The method of claim 1 wherein said second reference pressure isselected at a point within said preselected range wherein such referencepressure and the readout differ by the greatest amount.

3. The method of claim 1 wherein said reference pressure is from 40-60percent of said first reference pressure.

4. The method of claim 1 wherein said reflector means is moved in thedirection parallel and transverse to said source beam by moving saidBourdon tube relative to the detecting portion of said mirror deflectionsensing means.

5. The method of claim 1 including moving said light source with saidreflector means to preserve the spaced relationship therebetween.

6. The method of claim 1 including the additional step of determiningthe region of greatest error of said readout within said range byapplying a number successive incremental reference pressures within saidrange to said pressure gauge and recording the readout of each suchincremental reference pressure.

7. In a measuring instrument having a transducer capable of producing aphysical response functionally related in magnitude to the magnitude ofthe condition to be measured to produce a rotary deflection of areflector means, and having sensing means including a light sourcespaced from said reflector means for directing a source beam of light atsaid reflector means, and a light sensor for detecting the arc subtendedby the reflected beam from such reflector means, and a readout means forproducing a digital readout of the condition to be measured related tothe arc subtended between said source beam and said reflected beam, themethod of calibrating such an instrument for a substantially linearresponse over a preselected range of values measuring said conditionwhich comprises:

applying to the transducer a first reference condition having apredetermined magnitude corresponding to the upper limit of suchpreselected range;

detecting the arc subtended by the reflected beam from such reflectormeans as a result of application of such first reference condition;

moving said reflector means parallel to said source beam to change thesize of the arc subtended by said reflected beam on said light sensor toproduce an accurate readout corresponding to said first referencecondition;

applying to said transducer a second reference condition within saidrange wherein said readout differs substantially from said secondreference condition; and

moving said reflector means transversely to the direction of said sourcebeam while preserving the size of the arc subtended at said firstreference condition to produce an accurate readout corresponding to saidsecond reference condition.

8. The method of claim 7 wherein said second reference condition isselected at a point within said preselected range wherein such referencecondition and the readout differ by the greatest amount.

9. The method of claim 1 wherein said reference condition is from 40-60percent of said first reference condition. I

1. In a Bourdon tube manometer having a reflector means mounted on aBourdon tube pressure gauge, a mirror deflection sensing means includinga light source for directing a source beam of light spaced from saidreflector means at said reflector means and a light sensor for detectingthe arc subtended by the reflected beam from such reflecting means, anda readout means for producing a digital readout of pressure related tothe arc subtended between said source beam and said reflected beam, themethod of calibrating such a manometer for a substantially linearresponse over a preselected range which comprises: applying to suchBourdon tube pressure gauge a first reference pressure corresponding tothe upper limit of such preselected range; detecting the arc subtendedby the reflected beam from such reflector means as a result ofapplication of such first reference pressure; moving said reflectormeans parallel to said source beam to change the size of the arcsubtended by said reflected beam on said light sensor to produce anaccurate readout of said first reference pressure; applying to saidBourdon tube pressure gauge a second reference pressure within saidrange wherein said readout differs substantially from said secondreference pressure; and moving said reflector means transversely to thedirection of said source beam while preserving the size of the arcsubtended at said first readout pressure to produce an accurate readoutcorresponding to said second reference pressure.
 2. The method of claim1 wherein said second reference pressure is selected at a point withinsaid preselected range wherein such reference pressure and the readoutdiffer by the greatest amount.
 3. The method of claim 1 wherein saidreference pressure is from 40-60 percent of said first referencepressure.
 4. The method of claim 1 wherein said reflector means is movedin the direction parallel and transverse to said source beam by movingsaid Bourdon tube relative to the detecting portion of said mirrordeflection sensing means.
 5. The method of claim 1 including moving saidlight source with said reflector means to preserve the spacedrelationship therebetween.
 6. The method of claim 1 including theadditional step of determining the region of greatest error of saidreadout within said range by applying a number successive incrementalreference pressures within said range to said pressure gauge andrecording the readout of each such incremental reference pressure.
 7. Ina measuring instrument having a transducer capable of producing aphysical response functionally related in magnitude to the magnitude ofthe condition to be measured to produce a rotary deflection of areflector means, and having sensing means including a light sourcespaced from said reflector means for directing a source beam of light atsaid reflector means, and a light sensor for detecting the arc subtendedby the reflected beam from such reflector means, and a readout means forproducing a digital readout of the condition to be measured related tothe arc subtended between said source beam and said reflected beam, themethod of calibrating such an instrument for a substantially linearresponse over a preselected range of values measuring said conditionwhich comprises: applying to the transducer a first reference conditionhaving a predetermined magnitude corresponding to the upper limit ofsuch preselected range; detecting the arc subtended by the reflectedbeam from such reflector means as a result of application of such firstreference condition; movIng said reflector means parallel to said sourcebeam to change the size of the arc subtended by said reflected beam onsaid light sensor to produce an accurate readout corresponding to saidfirst reference condition; applying to said transducer a secondreference condition within said range wherein said readout differssubstantially from said second reference condition; and moving saidreflector means transversely to the direction of said source beam whilepreserving the size of the arc subtended at said first referencecondition to produce an accurate readout corresponding to said secondreference condition.
 8. The method of claim 7 wherein said secondreference condition is selected at a point within said preselected rangewherein such reference condition and the readout differ by the greatestamount.
 9. The method of claim 1 wherein said reference condition isfrom 40-60 percent of said first reference condition.